This invention relates to precision step-and-repeat alignment and exposure systems for projection printing of a pattern from a reticle onto a semiconductive wafer.
Step-and-repeat alignment and exposure systems are used to print a circuit pattern appearing on a reticle onto a semiconductive wafer having a photosensitive coating by repeatedly projecting an image of the reticle onto the wafer using an actinic light source. Since the image of the reticle will be used to generate each circuit on the semiconductive wafer, it is important that the reticle be free of dirt on the clear or transparent areas as well as free of defects or damage that would affect the projected circuit pattern. In the past, such reticles have usually been manually inspected by the system operator before they are placed in the system reticle holder. This procedure has the disadvantages that the operator may not notice dirt or damage that will cause the circuits to be defective, and dirt may appear or damage may be caused after inspection when the reticle is placed in the reticle holder.
There are also some inspection devices that are used to inspect the reticle before it is placed in the step-and-repeat alignment and exposure system. The reticle to be used is compared with a reference reticle known to be good by using two identical optical scanners to scan identical portions of the two reticles at the same time. If there is dirt or damage on one of the reticles, the output of one of the optical scanners will differ from that of the other and the location of the detected dirt or damage can be determined. The inspection device operator can then either clean, repair or replace the reticle, as necessary. One of the significant disadvantages of such inspection devices is that the reticle must be handled by the operator after it has been inspected so that it can be placed in the alignment and exposure system. This post-inspection handling can result in damage to or dirt on the reticle, thus defeating the purpose of the inspection. In addition, it requires an additional reticle as the reference reticle.
A system for on-machine inspection of a reticle has been proposed using a laser light scattering technique. This system has the disadvantage that it will detect dirt on both the clear and opaque areas of the reticle. Since dirt on the opaque areas will not affect the quality of the projected image, a "false alarm" will be given. In addition, this system will not detect defects or damage in the pattern on the reticle.